If all you want is to avoid the array copy you can:
$tmp = & $currentQueue;
$currentQueue = & $otherQueue;
$otherQueue = & $tmp;
But if you're really trying to optimize this code I would suggest finding a Breadth-First Search algorithm from Knuth or Cormen et al and implement it in PHP (or find an existing implementation).
Also be sure you actually need to optimize this code.